Electric motor control device



March 21, 1950 c. R. SCHAFER ELECTRIC MOTOR CONTROL DEVICE Original Filed Feb. 25, 1945 INVENTOR. CWT/5.9 f7. 5cflflFE/7 fl 770/7715 Y Patented Mar. 21, 1950 ELECTRIC MOTOR CONTROL DEVICE Curtiss It. Schafer, Rye, N. Y., assignor to Minnenfla spoils-Honeywell tor Company, Minneapolis, Minn., a corporation of Delaware Continuation of application Serial No. 579,253, February 25, 1945. This application June 13, 1946, ScrialNo. 67.6,400

20 Claims. 01. 318 29) 1 This invention relates to the field of instrumentation, and more particularly to instruments for giving a local or remote indication of change i in a condition to be measured.

Instruments of this type are not broadly new, and known instruments of this type can be divided into two general classes. The first class of instrument includes a condition responsive member known as a pickup element and an indicator in the form of a meter, the two being connected by a suitable electric circuit. In this type of instrument there is a continuous electrical output from the pickup device which varies in magnitude with change in the condition to be measured: the indicating instrument is responsive to this change in the magnitude of the signal from the pickup device.

The second broad class of instruments such as that comprising the subject matter of my invention differs from the first class in that the indicator comprises an index movable with respect to a fixed scale by a motor. In this type of instrument, the electrical circuit, generally a form of bridge, is such that it has a normal or balanced position, and any change in the condition to be measured, causing a change in the output of the pickup unit and unbalancing the circuit, results.

in operation of the motor to change the indication of the index and at the same time return the circuit to its normal or balanced condition.

Of the two types of instruments, the latter type is generally considered more accurate and stable in many applications, operating as it does about a normal condition. The advantages of bridge circuits are well'known, and the peculiar insensitiveness of normally balanced bridge circuits to variations in the applied voltage is an added advantage of this type of system.

Pickup units of various kinds have been used in instrumentation systems, but I am convinced that in the long run the most satisfactory condition responsive members are those based on a change in capacitance with change in the condition being supervised, and the pickup unit embodied in my invention is therefore of this class.

A major drawback in the use of capacity or condenser pickups lies in the fact that in instrumentation systems of the second type it is necessary that the motor be energized from a commercial source of alternating current. Conveniently constructed capacit pickups do not operate efliciently in impedance bridges at commercial or low frequencies, and the problem of integrating in a single system the desired low frequency motor rebalancing of a normally balanced bridge 2 circuit with the prefered high frequency energizetlon of the capacity pickup has not heretofore been successfully solved in a simple and economical manner.

It is an object of my invention to provide a satisfactory instrumentation system including a capacity pickup and a motor actuated indicator.

It is another object of my invention to provide means for controlling the operation of a low frequency motor in accordance with change in a high frequency circuit.

A more specific object of my invention is to provide an instrument in which a first normally balanced network or sensing bridge is energized with high frequency alternating current modulated at a power frequency, the bridge unbalance signal being thereafter demodulated and the demodulated signal being used in a second bridge to control the actuation of a motor indicator.

A more limited object of m invention is to provide a balanceable network which is responsively unbalanced by a signal voltage applied across an impedance member of the network, and which is rebalanced by changing the ratio between impedances of further members of the network.

Various other objects, advantages, and features of novelty which characterize my invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of my invention, its advantages, and objects attained by its use, reference should be had to the subjoined drawing, which forms a further part hereof, and to the accompanying descriptive matter, in which I have illustrated and described a preferred embodiment of my invention.

The single figure of the drawing is a schematic wiring diagram of an instrument constructed according to the principles of my invention, and shows a device particularly adapted for the measurement and control of pressure. However, it will be realized that my instrument is equally well adapted to the measurement and control of liquid level, temperature, or any other variable to change in which variation in the capacitance of the capacitor can be related.

Construction As shown in the drawing, my invention is seen to comprise certain major components including a modulated oscillator ill, a normally balanced network or sensing bridge ll, an asymmetrically conducting member in the form of a detector or demodulator l2, an impedance unit or control alternating voltage power frequency. The structure and function of these major components, and the electrical circuits combining them into a single operative instrument, will now be considered in further detail.

since modulated oscillators are well known in the art, I have not shown the detailed structure of member III, which may be energized from any suitable power supply such as self contained batteries. Member ID is shown as being provided with a pair of output terminals 28 and 2|, and a pair of input terminals 22 and 23 are also provided, to which the low frequency modulating voltage is supplied. This modulating voltage is provided, through a transformer 25 having a primary winding 24 and a secondary winding 28, from conductors i1. One suitable embodtment of an oscillator of this type is illustrated and described on pages 412 and 413 of the 1944 edition of Radio Amateurs Handbook, published by the American Radio Relay League, Inc., West Hartford, Connecticut. In this particular oscillator the envelope of the output wave is 180 out of phase with the modulating voltage, or in phase with the power frequency because of the phase reversal in transformer 25.

The function of member In is to supply electrical energy, at output terminals -2|l and 2|, in the form of a high frequency carrier voltage modulated at a lower frequency. A carrier frequency of 500 kilocycles per second and a modulating frequency of 400 cycles per second are suggested as serving satisfactorily.

Sensing bridge II is shown to comprise a pair of capacitors 40 and 4| and a pair of inductors 42 and 48, the latter comprising electrically identical secondary windings of an air-core transformer 44 havinga primary winding 45. Lower terminal of winding 42 and upper terminal 5| of winding 48 are connected to one output terminal I54 of bridge H by conductors 52 and 53, respectively. Capacitors 48 and 4| are connected to the other output terminal 55 of bridge II by conductors 48 and 49, respectively, and the bridge circuit is completed by conductors 58 and 59, connecting capacitors 48 and 4| with inductors 42 and 48, respectively. The sensing bridge is energized from oscillator ill by the actionof transformer 44: the upper terminal 56 of coil 42 and the lower terminal 51 of coil 43 may be considered the input terminals of the bridge.

The inductances of inductors 42 and 43 and the capacitance of capacitor 40 are fixed, while the capacitance of capacitor 4| is variable. Since the inductances of inductors 42 and 43 are equal, bridge II can be in a balanced condition only when the capacitance of variable capacitor 4| is equal to the capacitance of fixed capacitor 40. The circuit constants are chosen so that the capacitance of capacitor 4| is always less than that of capacitor 48. There is always therefore a signal voltage at the output terminals 54 and 55 of the bridge; this voltage has the same form as the input from oscillator III, and its amplitude depends upon the magnitude of the difference between the capacitances of capacitors 48 and 4|.

Capacitor 4| is shown as connected by a link 82 with a bellows 88 which is mechanically responsive to change in the pressure in a conduit II. The arrangement is such that for any particular pressure in the conduit there is a unique value oi capacitance of capacitor 4|, and there fore a unique condition of unbalance of bridge H. Ii the pressure in conduit 8| increases, bellows Bil expands, increasing the capacitance of capacitor 4|, and causing a decrease in the output signal or bridge ll. Similarly, if the pressure in conduit 8| drops below the standard, bellows G0 contracts under the influence of the ambient atmospheric pressure, decreasing the capacitance of capacitor 4| and causing an increase in the output signal of the bridge. It is thus apparent that the sensing bridge functions, in combination with sensing element 60, to give a continuous signal which varies in amplitude with variation in the pressure being measured: the signal is in the form of a periodic voltage having carrier and power frequency components, the former being in 180 phase relation with the power frequency.

Demodulator l2 comprises an electronic rectifler tube 68 acting as a diode detector. The tube includes a cathode 18, a plate ll, and a heater filament l2 suitably energized by conventional means not shown. The function of this demodulator will be given in more detail when the operation of the invention as a whole is set forth: briefly, the demodulator serves to derive from the bridge output voltage a voltage having a principal component of the power frequency, whose amplitude varies with variation in the amplitude of the bridge output voltage.

Control bridge I3 is shown to comprise a pair of fixed resistors and 8|, which form adjacent arms of the bridge, and a variable resistor 82 including a slider 88 movable along a winding 84 to divide it into portions and 86: portions .85 and 86 of winding 84 comprise the second pair of adjacent arms of the bridge and are connected to the first pair of adjacent arms at input terminals 88 and 9|. Slider 83 of variable resistor 82 and a common terminal 82 between resistors 80 and 8| comprise the output terminals of the control bridge. Bridge I3 is energized from conductors I! through a transformer I00 having a primary winding lfll and a secondary winding Hi2: transformer I00 derives from the source a voltage of the power frequency and of fixed amplitude and phase.

Since the resistances of resistors 8|| and 8| are equal. control bridge I! is in its balanced condition when slider 83 is in its zero or central position along resistance winding 84; that is, when the resistances of portions 85 and 86 of winding 84 are equal. Under these conditions, no voltage derived from transformer I00 appears at the output terminals of the control bridge. For all other positions of slider 88 a voltage of the power frequency derived from input transformer ||l|| appears at the output terminals of the bridge, the magnitude of this voltage varying with the amount of displacement of slider 88 from its zero position and its phase reversing when slider 88 moves from one side of its zero position to the other. In the embodiment of my invention shown in the figure the output voltage is out of phase with the input voltage or in phase with the power frequency source, due to the action of transformer I88, when slider 88 isabove its zero position.

A second, independent output voltage can appear at the output terminals of bridge ll. The output of bridge II is connected, through demodulator 88, between input terminal 88 and out-. put'terminal II of bridge II, and the current in portion 88 of resistor 82 due to this produces a voltage thereacross derivedfrom the unbalance voltage of bridge II: the derived voltage has an alternating component of the power frequency but of opposite phase whose amplitude varies with the amplitude of the bridge output. Since the derived voltage is applied to bridge I3 asymmetrically, a voltage of the same wave shape but of somewhat reduced amplitude appears at the output terminals ofthe control bridge if the latter is otherwise balanced, and this voltage is such that its power frequency component, as it appears between terminals 82 and 83 of bridge I3, is in phase with the power frequency source.

A consideration of the preceding paragraphs will make it evident that impedance unit I3 is affected by two different applied periodic voltages, and that these voltages will buck when slider 83 is below its center position and will boost when slider 83 is above its central position. It will also be apparent that it is possible to so select the circuit components that for any reasonable range of unbalance of sensing bridge II an equal and opposite unbalance of bridge I3 may be brought about by proper adjustment of slider 83. For any other position of slider 83, one or the other of the bucking voltages predominates, and a bridge output voltage having a component of the power frequency and of reversible phase is thus made available for use as will now be set forth.

Amplifier I4 is shown to comprise a pair of input terminals I08 and H and a set of output terminals III, H2, H3, and H4: a further pair of terminals I I and H8 is provided by which the amplifier may be energized from conductors I1. The function of amplifier I4 is to provide a first alternating current at output terminals III and H2, and a second alternating current at output terminals H3 and H4 in phase quadrature with the first current, the quadrature relation changing from a leading one to a lagging one with reversal in the phase of a signal voltage applied to the amplifier. Electrical energy for one of these currents is derived from conductors I'I directly, while that for the other phase is derived from conductors il through a control circuit, included in amplifier I4, which is responsive to the signal applied to the input of the amplifier. It will, of course, be appreciated that this input signal must have a component of the frequency of the source and either substantially in phase with the source or substantially 180 degrees out of phase in order for the device to operate satisfactorily.

Amplifier I4 may be of any one of a number of motor control amplifiers for regulating the direction of operation of a motor in accordance with the phase of a signal impressed on the input of the amplifier: one such amplifier is shown in Anschutz-Kaempfe Patent 1,586,233.

Motor I5 comprises a pair of field windings I32 and I33 so mounted as to be mechanically in quadrature, and a rotor carried by a shaft I40 for rotation with respect to the field windings. In actual practice a larger number of poles may be used in the motor stator. If the windings are energized by alternating currents which themselves are in quadrature, a rotary field is set up within the motor which causes rotation of the armature and with it shaft I40. This method of operation of the motor is familiar to those skilled in this art.

gearing I83, for causing movement of the slider with respect to winding 84. The output of reducing gearing I38 is also connected to a control device I6 which in the illustrated embodiment of my invention comprises a throttle valve regulating the supply of fluid under pressure to con- Shaft I40 of motor I5 is mechanically conduit GI. Operation of motor I5 is thereforeeffective to affect the balance of bridge I3 and simultaneously to regulate the pressure within conduit 0|.

Transformer 25 is energized from conductors I! through conductors 32 and 33. Oscillator I0 is energized from the secondary winding of transformer 25 by conductors 26 and 21. Transformer 44 is energized from oscillator I0 by conductors 46 and 41, and energizes sensing bridge II by transformer action. Transformer I00 is energized from conductors I! by conductors I03 and I04. Bridge I3 is energized from the secondary winding of transformer I00 by conductors I05, I06, and I5. Amplifier I4 is energized from conductors I1 through conductors II! and H8, and the output of bridge I3 is connected to the input of amplifier I4 by conductor I23 and ground connections 10 and I24. Winding I33 of motor, I5 is supplied with electrical energy directly from conductors II! and II8, by conductors I20 and I2I, and constitutes the line winding of the motor. A capacitor I25 is connected in series with this winding to cause a 90 degrees phase shift between the current flowing in winding I33 and the voltage of the source. Winding I32 of motor I5 is supplied with electrical energy of reversible phase, derived from conductors Ill and H8 under the control of am plifier I 4, by conductors I30 and I3I, and constitutes the amplifier winding of the motor. A capacitor I22 is connected in parallel with this winding and is chosen, with respect to Winding I32, so that the parallel circuit is nearly resonant at the power frequency: the exact size of this capacitor is chosen so that the currents in the two motor windings have a 90 degree phase angle.

The output circuit of sensing bridge II may be traced from its output terminal 54 through conductor I3, demodulator 68, conductors l4 and 75, input terminal 90 of bridge l3, portion 86 of winding 84, slider 83, and ground connections I6 and TI to terminal 55 of sensing bridge II. The demodulated output of sensing bridge II is accordingly impressed upon portion of winding 84 of control bridge I3 so that whenever sensing bridge II is not balanced an extraneous voltage is unsymmetrically inserted into the control bridge circuit.

Operation balanced, as previously set forth, and an output voltage appears between terminals 54 and 55 of bridge II. This output voltage is applied to the circuit including demodulator I2 and portion 86 of winding 84. During half cycles of the carrier voltage when plate II is negative with respect to cathode 10, no current flows in the circuit just recited, while durin the half cycles of the carrier voltage when plate ll is positive of an output signal between terminal 32 and slider 83 which is accordingly impressed upon the input of amplifier I4. This signal takes the form of the upper half of the modulated carrier wave. whose positive peaks are in phase with those of the voltage energizing bridge l3 and whose envelope has a zero phase angle with the power frequency.

- As is well known, an irregular wave form of the tim just referred to can be analyzed into a unidirectional component and a number of components of various harmonic frequencies. This particular wave form, in addition to its unidirectional component, has a principal alternating component whose frequency is the same as that of the voltage supplied .by conductors II, with which the component is in phase. Although all of the alternating components of the wave form can be transmitted through amplifier I4, it may be desirable to design amplifier ll to suppress the higher harmonics to a preferred extent. In any case, motor I5 is especially designed for operation from a source whose frequency is that supplied by conductors l1: components having other frequencies, as well as the unidirectional component, have principally-a braking effect on the motor. Although the motor does not run with the speed and efilciency it would show if both windings I32 and I33 were energized directly from conductors I1, the motor does run with somewhat reduced efficiency, and the direction in which the motor runs is such as to move slider 83 in a downward direction- The efficiency of motor I5 is improved by the provision of capacitor I22 which tunes the parallel circuit more nearly to resonance at the power frequency. The impedance of the parallel circuit to this component of the complex wave is accordingly much higher than its impedance to other components. and a larger proportion of driving energy is absorbed from the selected component than would otherwise be the case.

Movement of slider 83 is effective to accomplish sistance in the output circuit of bridge II is reduced, thereby reducing the actual value of the voltage drop through this resistance. In the secnd place, bridge I3 is itself unbalanced so that a second voltage appears between terminal 32 and slider 83: this voltage is out of phase with the principal component of the voltage derived from sensing bridge II, and therefore reduces the effectiveness of that component in actuating mofor IE. Continuous operation of the motor continues to move slider 33 until a position of the slider is reached at which the voltage derived from transformer I03 is lust sufficient to counteract the useful component of the voltage derived from bridge II. When this takes place, operation of motor I5 is interrupted.

At the same time that slider 33 is being moved along winding 8|, operation of motor It causes, valve ii to be operated, thus varying the supply of fluid under pressure to conduit GI, and chansing the pressure in conduit ll. With each incre ment of change in pressure in conduit 8| a corresponding increment of change in capacitance of capacitor ll takes place, the voltage in portion 30 of winding 34 due to the rectified output or bridge II is incrementally reduced, and the opposing voltage due to displacement of slider 33 from its central position is increased. The unbalance of bridge I3 is accordingly reduced and this condition continues until valve I3 and slider 33 reach settings at which the pressure in conduit BI is constant, when a condition of balance between the demodulated output of sensing bridge II and the unbalance output of control bridge I: has been I two different functions. In the first place, the res reached. No voltage is supplied to amplifier it under these conditions, so operation of motor [5 ceases, and the system may be said to be in equilibrium, with bridge I3 in electrical balance and bridge I I in electrical unbalance.

It will be obvious that an initial rise of th pressure in conduit 6i has an effect on control bridge I3 and therefore on motor I5 opposite to that of a decrease in pressure. The demodulated voltage effective in control bridge l3 varies in magnitude with change in the condition being measured, growing larger when the pressure Increases and growing smaller when the pressure decreases. If for any given state of balance of bridge-I3 the pressure in conduit 8| changes in a first direction, the voltage derived from sensing bridge II is increased as compared with that instantaneously being derived from transformer I02, while if the pressure changes in the opposite direction the voltage derived from sensing bridge ti decreases as compared with that instantaneously being derived from transformer I02. This results in opposite energization of amplifie I with opposite change in the pressure in conduit GI, and accordingly reversible operation of motor I! takes place.

It must be realized that the form of my invention illustrated is adapted for maintaining only one particular pressure value in conduit GI, and that particular output voltages of transformers 25, H, and IN and oscillator III, as well as particular capacitances of capacitors 43 and 4|, impedances of windings 42 and 43, and resistances of resistors 33, 3|, and 32 can cooperate to regulate the pressure at only one value. If it is desired that the controlled pressure be regulable over a range, it is necessary that at least one of the components listed above (most conveniently the capacitance of capacitor 43) be made adjustable in addition to capacitor 41: this, however, is well within the ability of those skilled in the art after a perusal of the foregoing disclosure.

This application is a continuation of my copending application, Ser. No. 579,253, filed February25, 1945, now forfeited, and assigned to the assignee of the present application.

Numerous objects and adantages of my invention have been set forth in the foregoing description, together with details of the structure and function of the invention, and the novel features thereof are pointed out in the appended claims.

ponent; means independently supplying a second periodic voltage of fixed amplitude and phase, including at least a component of said power frequency; means deriving from said second voltage a voltage related to said power frequency component thereof; means deriving from said first voltage a voltage related to the power frequency component thereof; and means mechanically responsive to inequality between said derived voltages.

2. In a device of the class described, in combination: a first source of periodic voltage, of varying amplitude, including a carrier frequency component and a power frequency component; a second source of periodic voltage, of fixed amplitude and phase, including at least a component of said power frequency; an impedance device; means applying voltage from said second source across said device; means, including an asymmetrically conducting member, for deriving from said first source a voltage determined by the power frequency component of said first periodic voltage; and means mechanically responsive to inequality between said derived voltage and the voltage drop in a selected portion of said device due to said first applied voltage.

3. In a device of the class described, in combination: a first source of periodic voltage of varying amplitude, including a carrier frequency component and. a power frequency component; a second source of periodic voltage of fixed amplitude and phase, including at least a component of said power frequency; an impedance device; means applying voltage from said second source across said device; means, including an asymmetrically conducting member, applying voltage from said first source across a variable portion of said device; and means mechanically responsive to inequality between said second applied voltage and the voltage drop in said portion of said device due to said first applied voltage.

4. In a device of the class described, in combination: a first source of periodic voltage, of varying amplitude, including a carrier frequency component and a power frequency component; a second source of periodic voltage, of fixed amplitude and phase, including at least a component of said power frequency; an impedance device; means applying voltage from said second source across said device; means, including an asymmetrically conducting member, for deriving from said first source a periodic voltage having a component of said power frequency whose amplitude varies with the amplitude of said first periodic voltage; and means mechanically responsive to inequality between said second periodic voltage and the voltage drop in a selected portion of said. device due to said first applied voltage.

5. In a device of the class described, in combination: a first source of periodic voltage of varying amplitude, including a carrier frequency component and a power frequency component; a second source of periodic voltage of fixed amplitude and phase, including at least a component of said power frequency; an impedance device; means applying voltage from said second source across said impedance device; means, including an asymmetrically conducting member, for deriving from said first source a periodic voltage having a component of said power frequency whose amplitude varies with the amplitude of said first periodic voltage; and power frequency sensitive means mechanically responsive to inequality between said second periodic voltage and the voltage drop in a selected portion of said device due to said first applied voltage.

10 6. In a device of the class described, in combination: a first source of periodic voltage of variable amplitude, including a carrier frequency component and a power frequency component;

means varying the amplitude of said first periodic voltage in response to change in a condition; a second source of periodic voltage of fixed amplitude and phase, including at least a component of said power frequency; means deriving from said second source a voltage related in amplitude to the power frequency component thereof means deriving from said first source a voltage related in magnitude to the amplitude of the power frequency component thereof; and means mechanically responsive to inequality between said derived voltages.

7. In a device of the class described, in combination: means supplying a first periodic voltage of varying amplitude, including a carrier frequency component and a power frequency component; means independently supplying a second periodic voltage of fixed amplitude and phase, including at least a component of said power frequency; a voltage comparing impedance unit having an adjustable portion; means applying said second voltage across said impedance unit; means, including an asymmetrically conducting member, for deriving from said first voltage a voltage related to the amplitude of the power frequency component thereof; means mechanically responsive to inequality between said derived voltage and the voltage drop in a selected portion of said unit due to said first applied voltage, said last named means including a motor designed for energization with voltage of said power frequency; and means connecting said motor to said impedance unit for varying the size of said variable portion thereof, to make said derived voltage and said voltage drop equal.

8. In a device of the class described, in combination: a first source of periodic voltage of variable amplitude, including a carrier frequency component and a power frequency component; means varying the amplitude of said first periodic voltage in response to change in a condition; a second source of periodic voltage of fixed amplitude and phase, including at least a component of said power frequency; means deriving from said second source a voltage related in amplitude to the power frequency component thereof; means deriving from said first voltage a voltage related to the amplitude of the power frequency component thereof; and means, mechanically responsive to inequality between said derived volt ages, for causing change in said condition.

9. In a device of the class described, in combination: a first source of periodic voltage of varyin amplitude, includin a carrier frequency component and a power frequency component; a second source of periodic voltage of fixed amplitude and phase, including at least a component of said power frequency; an impedance device having an adjustable portion; means applying voltage from said second source across said impedance device; means, including an asymmetrically conducting member, for deriving from said first source a voltage related to the amplitude of the power frequency component thereof; means mechanically responsive to inequality between said derived voltage and the voltage drop in a selected portion of said unit due to said first applied voltage, said last named means including a motor designed for energization with voltage of said power frequency and means connecting said motor to said impedance unit for varying the 11 size of said variable portion thereof; and further means actuated by said motor for causin change in said condition.

10. In a device of the class described, in combination: a normally unbalanced electric bridge;

means energizing said bridge with a complex said second applied voltage and the voltage drop in a selected portion of said unit due to said first applied voltage.

11. In a device of the class described, in combination: a normally balanced electric bridge having impedance arms and input and output circuits; means energizing said bridge with alternating voltage of a selected frequency; a motor; means reversibly energizing said motor in accordance with reversible unbalance of said bridge; means impressing a signal voltage, having an alternatin component of said frequency, upon an impedance comprised in an arm of said bridge, whereby to cause voltage unbalance of said bridge; means varying the ratio of impedance arms of said bridge to cause equal and opposite voltage unbalance of said bridge; and means connecting said ratio varying means to said motor for actuation thereby.

12. In a device of the class described, in combination: first and second normally energized electric bridges havin impedance arms and input and output circuits; means varying the impedance of an arm of one of said bridges whereby to affect the output of said bridge; an impedance member comprised in an arm of the other of said bridges; means impressing the output of said one bridge upon said impedance, whereby to affect the balance of said ,other bridge; and means varying the ratio between arms of said other bridge to rebalance said other bridge.

13. In a device of the class described, in combination: first and second normally energized electric bridges having impedance arms and input and output circuits; means varying the impedance of an arm of one of said bridges in response to change in a condition, whereby to affect the output of said bridge; an impedance member comprised in an arm of the other of said bridges, means impressing the output of said one bridge upon said impedance, whereby to affect the balance of said other bridge; a motor;

means reversibly energizing said motor in ac- I cordance with reversible unbalance of said bridge; means varying the ratio between arms of said bridge to rebalance said bridge; means connectin said last named means with said motor for actuation thereby; and condition controlling means actuated by operation of said motor.

14. In a device of the class described, in combination: a plurality of impedance bridges having impedance arms and input and output circuits; a plurality of sources of alternating voltage; means energizing the input circuit of a first of said bridges from a first of said sources; means energizing the input circuit of a second of said bridges from a second of said sources; a first variable impedance member; means varying the impedance of said member in response to change in a condition; means connecting said member in an arm of said second bridge to vary the unbalance voltage thereof; means energizing one arm of said first bridge with said unbalance voltage, whereby to cause voltage unbalance of said first bridge; and means varying the ratio of impedance arms of said first bridge to cause equal and opposite voltage unbalance of said bridge, whereby said bridge is again brought to a condition of voltage equilibrium.

15. In a device of the class described, in combination: a plurality of impedance bridges having impedance arms and input and output circuits; a plurality of sources of alternating. voltage; means energizing the input circuit of a first of said bridges from a first of said sources; means energizing the input circuit of a second of said bridges from a second oi. said sources; a first.

variable impedance member; means varying the impedance of said member in response to change in a condition; means connecting said member in an arm of said second bridge to vary the unbalance voltage thereof; means energizing one arm of said first bridge with said unbalance voltage, whereby to cause voltage unbalance of said first bridge; and means varying the ratio of impedance arms of said first bridge to cause equal and opposite voltage unbalance of said bridge, whereby said bridge is again brought to a condition of voltage equilibrium; a motor; means connecting said motor in the output circuit of said first bridge for reversible energization by reversible voltage unbalance thereof; means connecting said motor with said ratio varying means for actuation thereof; and control means actuated by said motor.

16. In a device of the class described, in combination: a plurality of impedance bridges having impedance arms and input and output circuits; a plurality of sources of alternating voltage having different frequencies; means energizing the input circuit of a first of said bridges from a first or said sources; means modulating the voltage from a second of said sources by voltage from said first source; means energizing the input circuit of a second of said bridges with said modulated voltage, a first variable impedance member; means varying the impedance of said member in response to change in a condition; means connecting said member in an arm of said second bridge to atlect the balance thereof; means for demodulating said modulated voltage; means connecting said demodulatlng means in the output circuit of said second bridge; means energizing one arm of said first bridge with said 60 demodulated voltage, whereby to cause voltage unbalance of said first bridge; and means varying the ratio of impedance arms of said second bridge, to cause equal and opposite voltage unbalance of said bridge, whereby said bridge is again brought to a condition of voltage equilibrium.

17. In a device of the class described, in combination: a plurality of impedance bridges having impedance arms and input and output circults; a plurality of sources of alternating voltage having difierent frequencies; means energizing the input circuit of a first of said bridges from a first of said sources; means modulating the voltage from a second of said sources by voltage from said first source; means energizing the input circuit of a second of said bridges with said modulated voltage; a first variable impedance member; means varying the impedance of said member in response to change in a condition; means connecting said member in an arm of said second bridge to affect the balance thereof;

means for demodulating said modulated voltage;

means connecting said demodulating means in the output circuit of'said second bridge; means energizing one arm of said first bridge with said demodulated voltage, whereby to cause voltage unbalance of said first bridge; means varying the ratio of impedance arms of said first bridge to cause equal and opposite voltage unbalance of said bridge, whereby said bridge is again brought to a condition of voltage equilibrium; a motor; means connecting said motor in the output circuit of said first bridge for reversible energization by reversible voltage unbalance thereof; means connecting said motor with said ratio varying means for actuation thereof; and control means actuated by said motor.

18. In a device of the class described, in combination: means supplying a first periodic. voltage varying in response to a condition and including at least a carrier frequency component and a power frequency component; means independently supplying a second periodic voltage of fixed amplitude, including at least a component of said power frequency; a variable impedance pendently supplying a second periodic voltage of fixed amplitude, including at least a component of said power frequency; a variable impedance unit for comparing a voltage applied thereto in a first fashion with a variable portion of a voltage applied thereto in a second fashion; means applying said power frequency components to said unit for comparison thereby; a. motor designed for energization with voltage alternating at said power frequency; and means energizing said motor for operation when said components being compared are not equal.

20. In a device of the class described, in combination: means supplying a first periodic voltage varying in response to a condition and including at least a carrier frequency component and a power frequency component; means independently supplying a second periodic voltage of fixed amplitude, including at least a component of said power frequency; a variable impedance unit vfor comparing a voltage applied thereto in a first fashion with a variable portion of a voltage applied thereto in a second fashion; means applying said power frequency components to said unit for comparison thereby; a motor designed for energization with voltage alternating at said power frequency; means energizing said motor for operation when said components being com- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,039,405 Green et al May 5, 1936 2,183,725 Seeley Dec. 19, 1939 

